A charged particle beam such as a proton or a carbon ion is characterized in that, in the case where it is irradiated inside a body, the intra-body given energy (the amount of absorbed dose) per mass thereof becomes maximum at a position in the vicinity of the range terminal thereof. A particle beam irradiation apparatus utilizes this characteristic to concentrate a dose into a diseased site, so that the effect on the normal tissue around the diseased site can be suppressed to minimum. In general, the transverse cross section of the charged particle beam obtained from an accelerator is a circular or an elliptical shape having a diameter of approximately less than 1 cm. In contrast, in some cases, the size of a diseased site such as a tumor may be ten and several centimeters . Accordingly, as a method of uniformly irradiating a small charged particle beam obtained from an accelerator onto a large diseased site, there exists a so-called “scanning irradiation method” in which a charged particle beam is scanned over the diseased site.
Specifically, control is performed with a scanning electromagnet in such a way that a charged particle beam spot is scanned over the cross section of an irradiation target situated on a plane perpendicular to the irradiation direction of the charged particle beam spot, and the irradiation amount of a charged particle beam is controlled at each of the scanning positions. Eventually, the distribution of the total dose of charged particle beam spots at all scanned positions is made to coincide with the shape of a tumor as much as possible . The scanning of a beam spot in the depth direction is realized by changing the kinetic energy of the charged particle beam. In Patent Document 1, there is described a particle beam therapy system in which, by scanning a charged particle beam in such a way as described above, the three-dimensional distribution of doses is formed in such a way as to coincide with the three-dimensional shape of a diseased site.
In Patent Document 1, there are disclosed an irradiation apparatus and an irradiation method in which a proton beam, accelerated by an accelerator, having beam energy of approximately 200 MeV is formed by a scanning electromagnet in a direction perpendicular to the traveling direction of the beam and in such a way as to have an arbitrary-shape dose distribution. In this case, by changing the energy of a proton beam through insertion of a so-called “range shifter”, the proton beam is scanned also in the depth direction. In addition, as may be necessary, the size of a beam spot at an irradiation position is changed by inserting a scattering material such as lead in the irradiation system. The objective of making the size of a beam spot large is to make it possible that a sufficient dose distribution can be formed by less irradiation of beam spots onto a target region of a given size.